The purpose of this study is to determine the therapeutic efficacy of the exp-NK cell therapy in research participants with relapsed high risk MM [defined as gene expression profile (GEP) 70 gene score ≥0.66 and/or metaphase chromosomal abnormalities and/or high LDH ≥ 360U/L] by establishing the (near) complete response rate. Response rate will be compared to case matched historical controls (patients who relapsed on Total Therapy 2 or 3 with high-risk MM defined as above). Disease-free survival and overall survival will be captured but are not primary or secondary endpoints.